1. Technical Field
This invention relates to the field of image processing, and in particular the processing of video sequences.
2. Related Art
Digital video systems rely on the imperfections of human vision and perception to render a video sequence by transmitting a decreased amount of information. As a result the subjective quality of a digital video system can be difficult to predict, because it is unrelated to the objective degradation present in the rendered video sequence. At present, the only reliable method by which the subjective video quality can be assessed is to run subjective tests, where a number of people give their opinion regarding the quality of an observed video sequence. Subjective testing is expensive and time consuming, and additionally not suitable for all applications, for example real time assessment of service quality.
Furthermore, most objective models to assess the quality of digital video systems are designed to compare systems constrained by broadcast standards. This makes it easier to realign the two video streams as the degraded sequence has the same characteristics as the original sequence in terms of frame rate, frame size and the position of the frame within a restricted search window. If an objective model has to assess the quality of digital video systems in multimedia applications, different variables will have an effect and will need to be accounted for. For example, there is no indication about the image standard that can be expected at the output of the system to be tested: while the original sequence could be in a broadcast format, the degraded sequence might be CIF, QCIF, or any frame size and/or resolution chosen by the user of a multimedia application. Furthermore, the degraded sequence is likely to be presented in a software window, which could be located anywhere on a computer screen, with the user having control over the size and aspect ratio of the window.
One application that is commercially available at present, the Tektronix implementation of the JNDmetrix model, has solved this problem with the introduction of a calibration signal, which contains time stamp information. Though very reliable, this technique is invasive, because the time stamp adds some visual features and an important amount of movement to the sequence being tested. The timestamp is visible across the screen, which excludes the use of the system in any real time assessment of video services. Also, it can be argued that as the system has an influence on the image content it therefore affects the performance of the video system under test.